4.2 DISEÑO Y ESTIMACIÓN DE CARGA DE LOS MURETES
4.2.1 MURETES ANTE CARGA DE COMPRESIÓN AXIAL
Given the recent philosophical literature on the importance and value of integration, there is good reason to investigate the role of integration in invasion biology. In the philosophical literature, integration is seen as a an acceptable middle ground between unificatory but reductionist accounts of science and accounts of the disunity of science, such as Dupré (1993) (Brigandt 2013). Moreover, the fact that integration is problem-oriented instead of theory oriented is seen as a value which unification does not share (Plutynski 2013).
At a theoretical level, we have good reason to expect integration in the field of invasion biology, as it exhibits many of the hallmarks of integration cited by Plutynski (2013). For many years, invasion biology
management of invasions. Also, even though the problem of biological invasions is global, invasions are often studied locally. For example, even large scale ecological niche models such as GARP and MAXENT, which are sometimes used to predict invasions, rely on local environmental variables such as temperature and precipitation (Sobek-Swant et al. 2012). With a few exceptions, invasion research usually focuses on identifying the mechanism of an invasion, such as the factors which increase the competitiveness of the invader or the invasibility of the community. There is also a history of collaboration in the field, with researchers from different backgrounds contributing data, methodology and techniques particular studies of invasion. Even though some invasion biologists lament the fact that there is little collaboration in the field (Milbau et al. 2009, Davis et al. 2001), there are numerous examples of collaborative projects in invasion biology (Gurevitch et al 2011 Rejmánek & Richardson 1996; Richardson & Rejmánek 2004; Richardson and Pysĕk 2006; Milbau et al. 2009; Moles et al. 2012).
Many of these collaborations have been very successful. Scientists now have a much deeper and more extensive understanding of the mechanisms of invasion and its management. More importantly, some of these collaborations have also produced successful predictions. One of the most important of these collaborations is a series of papers by David Richardson and Marcel Rejmánek, on conifers (Rejmánek & Richardson 1996; Richardson & Rejmánek 2004). Richardson and Rejmánek originally studied invasive and non-invasive species of pines and showed that invaders shared three important traits: small seed mass, small juvenile periods and short intervals between seed crops (Rejmánek & Richardson 1996). Plants which exhibit these traits have a high “index of invasiveness” (number of invasive species/number of rare/ threatened species) (ibid). Small seed mass helps plants to disperse over long distances, without the aid of mutualists. It also helps them to compete against other plants because of the sheer number of seeds which germinate (ibid). Short juvenile periods and intervals between seed crops further increase a plant’s competitiveness as it reaches the reproductive stage earlier than its competitors and can proliferate more extensively (ibid). This is especially important in disturbed habitats, which are the kind of habitats most susceptible to invasion (ibid).
Rejmánek and Richardson used this very simple framework to identify species of pines which pose particular threats to ecosystems. For example, they identified P. radiata, P. contorta, P. halepensis, P. patula, P. pinaster as the five most invasive species of pine and also identified 14 species of pine which do not pose a threat to invasion. Richardson & Rejmánek’s work is not a one-off case study. Their framework
can be generalized to cover many gymnosperms and even some angiosperms. For example, the framework can be used to make predictions about other conifers and some woody angiosperms (Rejmánek and Richardson 2004). They state that “conifers are probably unequalled in their capacity to improve the robustness of predictions in plant invasion ecology” (p. 327), because of the simple framework which has been shown to yield successful predictions. Rejmánek and Richardson conclude that the dispersal of pine seeds from plantations into adjoining natural or seminatural habitats is a major threat, especially in the Southern Hemisphere. They also identify forests in Sweden as a potential danger zone, as it is susceptible to invasion by Pinus contorta, a North American pine. Another interesting point which they mention, is their surprise that a recent global assessment of criteria and indicators for sustainable forest management did not mention the invasiveness of many conifer species. It seems that based on their predictions measures should be taken to ensure that conifer invasions do not occur.
It seems that this project is respected in the scientific community, as it is widely cited and to my knowledge, there is no criticism of the methodology or conclusions of the studies. In fact, it is considered by some to be the most successful example of prediction in invasion biology (Lake, 2004). However, many of the scientists who argue for larger-scale conceptual frameworks in invasion biology, have described these kinds of predictive frameworks as too limited. For example, Milbau et al. (2009) state that it is not enough to explore mechanisms of plant invasions at a particular scale, but that generalizations should be made across different scales. Moles et al. (2012) think of local studies as idiosyncratic, state that even though they produce “seemingly clear results, the larger picture is one of idiosyncrasy and inconsistency” (p. 117).
Yet this point should be seen as a problem for invasion biology in general, rather than a criticism of a study within invasion biology. The same authors who put forth these criticisms also state that invasions are extremely heterogeneous (ibid), and acknowledge that invasions are very complex processes, which can be studied in many (at least 7) different dimensions (Milbau et al 2009). Moreover, potential invaders are extremely heterogeneous, so it is impossible to extrapolate traits across taxa. For example, it is meaningless to talk of small seed mass in the case of animal invaders. In fact, when studies have attempted to abstract these differences between organisms, the resulting generalizations have been criticized as “trivial” (Richardson & Rejmánek 2004).
In other words, the criticism of these types of projects is not that they are not generalizable, but that they are not generalizable enough. Yet I think that pushing beyond mid-level generalizations is not a useful move in invasion biology. I picked Richardson & Rejmánek’s work as an example, because it is one of the few documented cases which yielded successful predictions. In contrast, the large-scale frameworks cover more taxa, yet they have not resulted in precise and successful predictions.
8. Conclusion
Invasion biology can be characterized as a field with two different aims. The first is to understand and explain the fundamental nature of invasions and the second is to predict particular cases of invasion. The recent trend in invasion biology is the attempt to satisfy both aims, by creating unifying frameworks for invasion. The aim of this paper has been to raise some concerns about the feasibility of such a project. As invasions are extremely heterogeneous, for a framework to be truly unifying it cannot be based on factors which cause invasion. In order to unify invasions a framework must classify invasions in a different way, such as the invasion process. The problem then is that predictions must focus on causes, hence a unifying framework must give up the aim of prediction.
The point here is not to minimize the significance of unifying frameworks, as they are essential for providing explanations for invasions, both in terms of particular invasions and of the the nature of biological invasion as a whole. Instead, my aim is to show that these frameworks cannot accomplish both aims of invasion biology. The aim of prediction must be left to those integrative frameworks which aim for low to mid-level generalizations. In short, there can be a unified theory of invasion biology, but it will not be predictive.